leaking trim in shower cubicle

[gadapay]

hi,
first time on this site, so hello!
i'm repairing a shower cubicle, which has been leaking significantly around the metal trim that has been used, please see photo.
it's a small cubicle, it's a power shower, and there is an 'almost horizontal' shelf rebate below the shower head.
the area which has leaked and blown the underlying plywood is basically where the tiling meets the trim at the corner edges, it has then crept horizontal or descended from there and is mildewed as in pic. i've now stripped out all the ply associated damaged frame and replaced it with new frame and hardybacker board, silicon in all joints. at the point of tiling. the rest of the cubicle is completely fine so i am assuming correct grout and adhesive use.
it has been suggested to me that the failure arose because the metal trim was tacked on rather than bedded on a 'skim' coat of adhesive,
Question 1, is fixing the trim on a skim of adhesive the best way to go? does this provide a worthwhile additional barrier?
Question 2, to my mind a shelf in a power shower enclosure is not a good idea but some people are insistent regardless! given that, is it a good idea to refit the same style trim leaving a 2mm gap between trim and tile to be filled with silicon?

if anyone has any better ideas to solve leaking corners whilst retaining a smart finish looking edge i'd very much welcome your suggestions.
thanks in advance for any helpful responses.
cheers!

[royaloakcarpentry]

Well I would silicone the internals on the recess, as a matter or course. You could leave a 2mm gap between tile and trim to be backfilled with silicone or alternatively backfill trim with silicone, fit tiles and wipe away excess which is forced out.

No matter what precautions you take, the client may have trouble down the line anyway. A recess left with pools of water on it after showering will not last long before the water penetrates the grout joints. grout is only resistant to water and not actually water proof, same goes for adhesives. Any 'water proof' grout/adhesives are only stated as such because they have a minimal moisture content after 15 minutes (normally). Would be far better to use an epoxy grout.

If there is a future leak then in my experience of leaks where hardibaker or other tile backer boards have been used, you still end up ripping the board out as they get knackered just the same as ply or plasterboard.

I take it you are 100% that it is not the pipe work for the shower which is leaking?

[gadapay]

ok thanks for that, yeah it's definitely not pipework leaking, just thought i'd ask in case i was missing something obvious. i do like the idea of bedding the trim on silicone , that makes sense. thanks a lot.

[royaloakcarpentry]

You still bed the trim onto adhesive.

You backfill the trim partly with silicone before putting the tile in and then wipe away the excess which is forced out by siting of the tile.